1. Field of the Invention
The present invention relates to a novel alloy having a variable spectral reflectance and an information recording material making use of the alloy.
More particularly, the invention is concerned with an alloy the crystal structure of which is varied in response to a light or heat energy applied thereto, resulting in a spectral reflectance variation which is utilized for various purposes such as recording of information, displaying and sensors.
2. Description of the Prior Art
In recent years, various methods have been developed for the recording and regenerating of informations, in view of the demand for higher recording density and current trend for digital recording. Among these systems, a system called optical recording disk system, which makes use of energy of laser beam for recording, erasing and regeneration of information, is considered as being promising because it offers a higher recording density than the conventionally used magnetic disks. A regeneration device which makes use of laser beam has been already put into practical use as a compact disk (CD) system. On the other hand, the recordable systems are broadly sorted into two types: namely, write-once type and reloadable type. The former type permits the writing of information only once, and is not capable of erasing the written informations. On the other hand, the later type allows repetitional recording and erasing. In the write-once type recording system, the recording portion of the recording medium is destroyed or deformed in such a manner as to form convexities and concavities by the energy of the laser beam applied thereto, and the regeneration of the information is conducted by detecting a change in the rate of the reflected laser beam caused by interference due to presence of the convexities and concavities. Usually, the recording medium of the write-once type is made of tellurium (Te) and its alloys. When such a material is used, the convexities and concavities and are formed by making use of melting and sublimination of the alloy by the energy of the laser beam. This type of medium, however, involves a problem concerning toxicity.
The reloadable system usually employs an optomagnetic material as the material of the recording medium. In this type of system, the recording is conducted by making use of magnetic anisotropy which occurs when the material is heated by light energy to a temperature around the Curie point. The regeneration of the information is carried out by detecting a difference in polarization angle caused due to both the magnetic Faraday effect of polarized incident light in the recording medium and magnetic force effect. This system is considered as being most promising, and intense studies have been made to put this system into practical use within several years. Unfortunately, however, no material is available which would provide sufficiently large difference in the polarization angle. Although various proposals have been made such as a multi-layer film, such proposals still further from problems of small S/N and C/N ratios. Another known reloadable type system makes use of the change of reflectance occurring due to a reversible phase change of the recording medium between amorphous structure and crystal structure. That is, the recording and reproduction is conducted by making use of a change in the reflectance which is caused by a reversible phase change between the amorphous and crystalline states of the material. An example of the materials suitable for use in this type of system is an alloy which is prepared by adding small amounts of germanium (Ge) and tin (Se) to tellurium oxides (TeO.sub.x). This system, however, suffers from a problem in that the stability of the phase at room temperature is low due to a low crystallization temperature of the amorphous phase, resulting in an inferior reliability of the disk.
In another example of the system which makes use of the reversible phase change, the recording and regeneration are conducted by means of a change in the color tone which is caused by a phase change between two crystalline states. This system utilizes, for example, an alloy which is disclosed in Japanese Unexamined Patent Publication No. 140845/1982. More specifically, this alloy has a composition which essentially consists of 12 to 15 wt % of Al, 1 to 5 wt % of Ni and the balance substantially Cu, and exhibits a change in the color from red to gold when the temperature thereof varies across the martensite transformation temperature. The martensite transformation is caused without fail when the temperature comes down. Therefore, the color which is obtained when the material is held at a temperature above the martensite transformation temperature cannot be maintained when the temperature has come down below the martensite transformation temperature. Conversely, the color which is obtained when the material temperature is below the martensite transformation temperature is changed to another color when the material is heated to a temperature above this transformation temperature. It is possible to record information by causing a local change of color in the recording medium.